1. Field of the Invention
The present invention relates generally to an antenna, and more particularly to an array antenna for a wireless communication device.
2. Description of Prior Art
Antenna gain is a measure of the ability of the antenna to receive and transmit wireless signals towards a particular direction. Generally speaking, the gain of the antenna mainly depends upon the size of the antenna, the radio frequency at which it operates and the efficiency with which it focuses the radio waves. A helical antenna may have high gain in the present market, but manufacture of this kind of antenna is complex, this antenna needs more accessories and the precision requirement to the dimension is strict so that the quality of the antenna may be difficult of assurance.
In order to increase the antenna gain, lots of antenna units may be arranged at regular intervals to form a radiation system, namely an array antenna. Owing to omni-direction of antenna used in Wireless Local Area Network (WLAN), the researcher needs to concern what kind of antenna unit will be chosen and how to arrange the antenna units. Those skilled in the art may all know that dipole antenna is omnidirectional, thus a dipole antenna will be chosen as an antenna unit in omnidirectional radiation system.
U.S. Pat. No. 6,014,112 issued on Jan. 11, 2000 and entitled “SIMPLIFIED STACKED DIPOLE ANTENNA” discloses an array antenna formed by four dipole antennas. The antenna array is a 75Ω system and operates at 750 MHz. A feed line of the antenna array is formed by metal patterns having a plurality of pairs of adjoining quarter wave resonant sections formed by different widths of the patterns and dipoles respectively coupled to the junctions of the pairs of quarter wave sections. The performance of the antenna array depends mostly on the size of the feed line of the metal pattern. However, the construct of metal pattern is so complicated that manufacture of the antenna array is inconvenient.